(1) Field of the Invention
The present invention relates to an electrophotographic toner. More particularly, the present invention relates to an electrophotographic toner having an optimum combination of the fixing property, offset resistance and durability.
(2) Description of the Related Art
A toner is used for developing a charged image in the field of the electrophotographic reproduction or printing. In general, this toner is formed by incorporating a colorant or a charge controlling agent into a binder resin and adjusting the particle size to a predetermined level.
In developing a charged image, the toner is mixed with a magnetic carrier to form a two-component developer, and a magnetic brush of this developer is formed on a developing sleeve having magnetic poles disposed in the interior thereof. This magnetic brush is brought into sliding contact with a photosensitive material carrying a charged image thereon to form a toner image. The formed toner image is transferred onto a paper sheet from the surface of the photosensitive material, and the toner image is fixed on the paper sheet by contact with a fixing hot roller.
Various physical properties have been proposed for the binder resin for the toner. For example, Japanese Unexamined Patent Publication No. 56-16144 discloses a powdery developer comprising a binder resin component composed of a polymer synthesized from a vinyl monomer or a mixture of such polymer, which has a chromatogram determined by the gel permeation chromatography, in which at least one peak value of the molecular weight appears in regions of 10.sup.3 to 8.times.10.sup.4 and 10.sup.5 to 2.times.10.sup.4.
Furthermore, Japanese Unexamined Patent Publication No. 60-3644 discloses a toner composition consisting essentially of a binder resin and additives, wherein the binder resin comprises (A) a component having a weight average molecular weight higher than 500,000, (B) a component having a weight average molecular weight of 20,000 to 200,000 and (C) a component having a weight average molecular weight of 1,000 to 20,000.
According to these proposals, by making a high-molecular-weight component and a low-molecular-weight component present in the binder resin for a toner, the blocking resistance, impact resistance and offset resistance are improved while maintaining a good low-temperature fixing property.
However, with recent increase of the copying speed in a copying machine and reduction of the power consumption, when the conventional binder resins for a toner are used, such troubles as insufficient fixing, increased occurrence of the offset phenomenon and shortening of the life of the toner arise, and no effective means for solving these problems has been developed.